Exhaust gas from an internal combustion engine such as an automobile engine contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like, which is generally released into the atmosphere after purification with an exhaust gas purifying catalyst capable of oxidizing CO and HC, and at the same time, reducing NOx. Regarding exhaust gas purifying catalyst, a three-way catalyst is known in which a noble metal such as platinum (Pt), rhodium (Rh) and palladium (Pd) is supported on a porous metal oxide support such as γ-alumina.
The metal oxide support may be formed of various materials, but in order to obtain a large surface area, generally alumina has been used. However, in recent years, in order to accelerate the purification of an exhaust gas by using chemical properties of the support, it has been proposed to use various other materials such as ceria (CeO2), zirconia (ZrO2) and titanium (TiO2) in combination with, or not in combination with, alumina.
For example, Kokai (Unexamined Japanese Patent Application) No. 9-925 discloses a catalyst particle having stacked therein a plurality of support material layers and specifically proposes, for example, a catalyst support particle having, in the periphery of a support particle of alumina, zirconia or silica working out to the core, a first layer comprising such a support material, on which platinum is supported, a second layer covering the first layer and comprising ceria, barium oxide or lanthanum oxide, a third layer covering the second layer and comprising a support material such as alumina, on which rhodium or lanthanum is supported, and a fourth layer covering the third layer and comprising a support material such as alumina, on which cerium, cobalt or the like is supported. According to Kokai No. 9-925, such a catalyst support is supposed to exhibit excellent NOx purifying performance when used as an exhaust gas purifying catalyst while supplying a reducing agent such as light oil.
In order to absorb the fluctuation of oxygen concentration in the exhaust gas and increase the exhaust gas purifying ability of the three-way catalyst, ceria having an oxygen storage capacity (OSC) of storing oxygen when the oxygen concentration in the exhaust gas is high, and releasing oxygen when the oxygen concentration in the exhaust gas is low, is used in the exhaust gas purifying catalyst.
In order to allow for efficient progress of oxidation of CO and HC and reduction of NOx by the activity of the three-way catalyst, the air-fuel ratio in the internal combustion engine must be a theoretical air-fuel ratio (stoichiometric air-fuel ratio). Therefore, a material having OSC is preferably used to absorb the fluctuation of oxygen concentration in the exhaust gas and maintain the oxygen concentration in the vicinity of the theoretical air-fuel ratio, so that the three-way catalyst can exert its exhaust gas purifying ability.
Various studies are being made on the use of ceria in an exhaust gas purifying catalyst, and it is also known to increase the OSC and heat resistance by forming a solid solution of ceria and zirconia (see, Kokai No. 2003-126694). For example, in Kokai Nos. 8-131830 and 2003-117393, a technique is proposed in which loading platinum on a support particle such as alumina and coating ceria or a ceria-zirconia solid solution on the platinum, thereby suppressing movement of the platinum during use of the catalyst as well as sintering caused by the movement.
According to recent studies, ceria is found to have not only OSC, but also by virtue of its strong affinity for a noble metal, particularly for platinum, the capability of preventing particle growth (sintering) of the noble metal supported thereon. When platinum is sintered during use of an exhaust gas purifying catalyst, active sites of the catalyst decrease and in turn, the efficiency in the oxidation and reduction of NOx is depressed. For this reason, it is very important to suppress the sintering of platinum.
In this way, ceria is important in terms of preventing platinum from sintering. However, ceria may undergo sintering, and the ceria itself sometimes becomes sintered, and thereby reduced in the effect of preventing sintering of platinum.
To solve this problem, Japanese Patent No. 3,262,044 discloses a catalyst support where at least either one particle of ceria and zirconia and an alumina particle are dispersed in a highly uniform manner, and a production method thereof. In Japanese Patent No. 3,262,044, such a catalyst support is obtained by mixing a salt solution containing at least either one salt of cerium and zirconia and an aluminum salt with an alkaline solution in a short time. In this catalyst support, at least either one particle of ceria and zirconia and an alumina particle are dispersed in a highly uniform manner and sintering of at least either one particle of ceria and zirconia is suppressed by an alumina particle between those particles.
According to the catalyst support particle and the production process thereof described in Japanese Patent No. 3,262,044 supra, sintering of ceria is effectively suppressed by the use of alumina. Under these circumstances, an object of the present invention is to provide a better catalyst support particle similarly by using alumina in a ceria-based catalyst support. Another object of the present invention is to provide a production process of the catalyst support particle and an exhaust gas purifying catalyst produced by using the catalyst support particle.
In the case of obtaining an exhaust gas purifying catalyst by loading platinum on this catalyst support particle, platinum cannot be loaded selectively on ceria and a large amount of platinum is supported also on alumina. The sintering preventing effect of ceria cannot be exerted on such platinum supported on alumina. Accordingly, still another object of the present invention is to provide an exhaust gas purifying catalyst capable of successfully exerting a sintering preventing effect of ceria on platinum while effectively suppressing sintering of ceria by the use of alumina, and a production process thereof.